Researchers Test 'Composite Pain Scale' for Horses

If only horses could speak up and let us know when something hurts, right? They can.

If only horses could speak up and let us know when something hurts, right? Guess what: They can.

Dutch researchers recently learned that, although horses can’t speak, they do have distinct ways of communicating pain. It’s just up to us to learn to recognize them.

Thijs van Loon, PhD, of Utrecht University in Utrecht, the Netherlands, and his fellow researchers have developed what they call the “composite pain scale,” or CPS. The CPS lists common pain behaviors that could be seen in most horses suffering from either abdominal or musculoskeletal pain and includes a few physiological parameters such as breathing frequency, intestinal sounds, and rectal temperature.

“The difficulty with evaluating pain in horses is that they cannot tell us how much pain they’re in or where it hurts,” van Loon told The Horse. “And that’s complicated by the fact that horses are prey animals, and they will naturally tend to hide their level of pain as a survival mechanism.”

Specific behaviors in the scale include restlessness, sweating, kicking the abdomen, pawing, weight shifting, muscle tremors, head turning (especially toward the abdomen), poor appetite, aggression towards humans, and a lack of response to humans. While some of these behaviors might point toward one kind of pain, the goal of the CPS is to become a global scale that encompasses any kind of pain, he said.

Pain is subjective and has a “strong emotional component,” van Loon said. This means individuals will respond differently to the same pain source. So while it’s impossible to develop a pain scale that is 100% accurate for each horse, the CPS nonetheless addresses the most common pain-related behaviors, he said.

Van Loon tested the CPS on 48 horses undergoing emergency colic surgery. Observers scored horses on the CPS every four hours for three days following the surgery.

After reviewing their results, van Loon and colleagues found that not only were the scores consistent from one observer to another, but they also were linked to the horse’s outcome. Horses with higher CPS scores were more likely to need a second surgery or be euthanized than horses with lower CPS scores, he said.

The CPS is a starting point for studying expressions of pain in horses, he said. Further research should lead to CPS charts specific to certain types of pain.

Van Loon’s CPS is derived from another CPS pain scale originally developed by French and Canadian researchers for acute musculoskeletal pain. That scale was published in 2008 under the leadership of Eric Troncy, DVM, PhD, of the University of Montreal Veterinary School.